No Display on HDMI Screen

armStoneA9r2 comes with LVDS and HDMI, other boards have RGB-LCD as well. In this case only the LVDS port is active.
To display an image via HDMI, you need to activate the HDMI port.
This can be done in the Device-Tree. Specific file: arch/arm/boot/dts/armstonea9r2q.dts in Linux source code.
There you can find the line